Our network room is scheduled to have a power down this evening, and I was wondering whether there were any steps we need to take to prepare for this? Or, will the system just come back up normally after a power shut off?

I tested with one of our backup studios by unplugging the mix engine this morning and it took about 3 minutes for the system to reboot.

When a node power cycles, it comes up the way it went down.
When an Engine power cycles, it will come up blank (no inputs) until it is instructed by Element to load which sources.
Engine VMIX channels (if in Engine Control state) will load last saved settings.
When Element power cycles, it loads whichever show profile it is configured to load as default. This will in turn load sources into the Engine.

Switch will load the configuration that has been saved in Startup-config. So if you haven't copied running-config to startup-config, you will likely have an unconfigured switch after power up. Always good to log into switch and verify what is the startup-config (show startup-config) if you are not sure this has been done.

Those are the basics, add them up and that should tell you how the system will return.